This invention relates to infrared transparency films and films for thermal imaging processes, and in particular, to a coating material for such films.
Infrared imaging involves the use of a focused infrared lamp to heat an infrared absorbing image, commonly referred to as the "original", which image is in contact with a substrate, i.e. a transparency film, having thermally sensitive imaging chemicals. Upon absorbing the focused infrared light, the infrared absorbing image heats the thermally sensitive imaging chemicals on the substrate, causing a chemical reaction, resulting in a copy of the original image on the substrate.
It is frequently desirable to prepare projection transparencies from originals which are actually plain paper copies prepared from electrophotographic imaging processes. Because the localized heating of the image results in partial remelting of the toner powder on the original, the original frequently adheres to the transparency film. When the original is separated from the transparency film, toner powder from the original is transferred to the transparency film. This transfer of toner powder reduces the density of the image on the original and may destroy the quality of the image. Thus, the original can be damaged when a transparency is made from it. The adherence of the toner powder to the transparency film may also result in undesirable effects in the transparency. When the image on the transparency film is black, the toner powder does not harm the image itself, but the powder may rub off the transparency film or transfer to surfaces coming in contact with the transparency film. When the image on the transparency film is a color, the toner causes the image to have irregular black spots in the colored image area. This is a major defect in the transparency.
A barrier film interposed between the transparency film and the original can prevent toner powder from being picked up and retained by the transparency. In a common color transparency, a film containing an acid does serve as such a barrier.
Coatings that are capable of preventing unwanted materials from adhering to a surface are known. McCown, U.S. Pat. Nos. 3,995,085 and 3,944,527 disclose hybrid copolymers consisting essentially of fluoroaliphatic radical containing acrylates or methacrylates, lower alkyl acrylates or methacrylates, and at least two polyalkyleneoxide acrylates and methacrylates. These hybrid copolymers are useful for treatment of fabrics and provide an improved balance of properties. They provide stain repellancy and soil release, with good resistance to crocking.
A matte finish surface incorporating fine particles of inorganic materials such as silica, magnesium oxide, titanium dioxide, or calcium carbonate, or organic materials such as polymethyl methacrylate or cellulose acetate propionate has been used to reduce adhesion (See Akman, U.S. Pat. No. 3,854,942.)
Another method for reducing adhesion to a photographic, hydrophilic surface is disclosed in British Patent Specification No. 1,477,409, assigned to Fuji Photo Film Co. Ltd. This reference discloses a process of surface treatment of a hydrophilic surface layer of a photographic element which process comprises applying a liquid comprising at least one organic fluoro compound thereto, wherein the liquid contains no film-forming polymer. This process improves the anti-adhesive property of a photographic element.
Another method is disclosed in Williams, et al, U.S. Pat. No. 4,321,404, in which a radiation curable adhesive coating composition comprising a polyfluorinated acrylate compound, a polyethylenically unsaturated crosslinking agent, and a film-forming organic polymer is applied to image transfer systems.
None of the foregoing disclosures are adaptable to the area of production of projection transparencies from plain paper copies.
Polymers useful for textile treatment, e.g. a copolymer of C.sub.8 F.sub.17 SO.sub.2 N(C.sub.2 H.sub.5)C.sub.2 H.sub.4 O.sub.2 C(CH.sub.3).dbd.CH.sub.2 and tetraethylene glycol dimethacrylate-hydrogen sulfide prepolymer prepared in accordance with the method of Erickson, U.S. Pat. No. 3,278,352, Example I, should be soft and have shear modulus of under 10.sup.7 dynes/cm.sup.2. These polymers, when coated upon a transparency film substrate, do not release toner powder completely. To be useful for transparency films, polymers should have a shear modulus over 10.sup.7 dynes/cm.sup.2. Sward hardness over 40, and preferably over 50, is necessary in order to have such modulus values and to release toner powder while maintaining a smooth, non-light scattering surface. Moreover, polymers useful for textile treatment are in the form of latices and do not coat smoothly at the low coating weights needed for transparency films.
Matte surface films are undesirable for projection transparencies because matte surfaces scatter light, consequently reducing the light reaching the screen. This reduction of light is detrimental to the quality of the projected image. Thus, the use of silica or other particles in a transparency coating detracts from the quality of the transparency.
The fluorochemical liquids disclosed in British Patent Specification No. 1,477,409 are unsuitable for coating transparencies because they migrate to the surface of the film and result in a greasy, low molecular weight film on the surface of the transparency. When they are applied directly to the surface of the transparency film, they are easily removed and smudged by handling.
Williams, U.S. Pat. No. 4,321,404 discloses a process in which monomers are applied to surfaces and cured with radiation to form polymers. Because the transparency films have coatings which are sensitive to high energy radiation, i.e. infrared, high energy ultraviolet, electron beams, such curing causes premature darkening of the films as the monomers are curing. If the monomers are polymerized before being applied to the transparency film, they become insoluble and cannot be coated from solutions.
It is an object of the present invention to provide an infrared transparency film and thermally imageable film which can be used to copy plain paper copies while repelling toner powder from the plain paper copies.
It is another object to provide a coating for infrared transparency film or thermally imageable film which prevents toner powder from plain paper copies from adhering to the film.
It is a further object to provide a coating for infrared transparency film or thermally imageable film which does not interfere with the chemicals in the thermally imageable layer.